The present invention relates to an improved apparatus and method for use in a focus and alignment system for aligning and focusing an image of a reticle onto a semiconductor wafer.
Current alignment systems such as described in use in U.S. Pat. Nos. 4,425,037 and 4,391,494 take advantage of the fact that reasonably good imaging is obtained in the visible part of the spectrum (546 to 580 nm) with a Wynne-Dyson corrected for optimum imaging at 405 and 436 nm. Because of longitudinal color, the visible image is defocused five to eight microns. Although this would be a detriment to obtaining excellent imaging in the visible range, it can be accommodated in a system which only serves to align the reticle image on the wafer at visible wavelengths.
This fortuitous circumstance no longer pertains when the corrected spectral range of the Wynne-Dyson is further away from the desired alignment wavelengths. For example, a DUV (248 nm) Wynne-Dyson might exhibit 50 microns or more longitudinal color in the visible range.
The problem can be avoided by aligning at the exposure wavelength, but this can result in two serious disadvantages. The alignment operation partially exposes the pattern before it is aligned, and in some cases the resist through which the alignment mark on the wafer must be seen is opaque at the exposure wavelengths, making alignment impossible. Thus, there is need to have a Wynne-Dyson system corrected over two wavelength ranges; one corresponding to the exposure spectrum and the other to the alignment spectrum.